The present invention relates to radio telecommunication networks. More particularly, and not by way of limitation, the present invention is directed to a Base Station System (BSS) and method for compensating traffic channel signal strength measurements for improved handover decisions in a cellular radio telecommunication network.
In cellular telecommunication networks operated in accordance with the Global System for Mobile Communications (GSM) standard, mobile stations measure and report the signal strengths of the currently serving cell and neighboring cells. The neighboring cells are identified in a neighbor cell list broadcast by the serving BSS. When a particular mobile station is engaged in an ongoing circuit-switched call, the mobile station measures the signal strength of the active traffic channel being used for the call, and measures the signal strengths of the neighboring cells' broadcast channels. These measurements are reported to the serving BSS in measurement reports. As the mobile station approaches the border of the serving cell, a decision is made as to whether to handover the mobile station to a neighboring cell by comparing the signal strength of the active traffic channel with the reported signal strengths of the neighboring cells' broadcast channels.
For network planning purposes, the cell border between two cells is defined as the locations where the signal strengths of the broadcast channels (BCCHs) from the two cells are equal, as adjusted by different offsets and hysteresis values. If the radio propagation characteristics of certain traffic channels in a cell significantly differ from the radio propagation characteristics of the BCCH of the same cell, inaccuracy is introduced because the cell border cannot be kept at the same location for incoming and outgoing handovers. This may cause a variety of unwanted effects and network performance degradation. For example, a mismatched border may cause premature or delayed handovers, ping-pong effects (mobile stations being repeatedly handed over back and forth between two cells), lower quality connections, and increased dropped calls.
A conventional solution for minimizing the unwanted effects and network performance degradation is to compensate for the difference in the radio propagation characteristics between traffic channels and the BCCH with a unique offset value that is added to or subtracted from the reported signal strength of the active traffic channel. However, the only accurate way to currently determine the value of the offset is through a trial-and-error tuning process. In this process, an optimizer estimates an appropriate compensation offset value by calculating the pathloss differences that apply to the signal passing through different cables, antennas, and the like, and finally transmitting on different frequencies. This value is applied to the traffic channel, and then statistics are gathered to determine the effectiveness of the compensation offset value. If the value was not correct (which is very likely), a new value is chosen, and statistics are again gathered to determine the offset's effectiveness. This is a time-consuming and error-prone method. Each cell's offset must be tuned to the cell's radio propagation differences between channels. However, the radio propagation characteristics vary between different cells and environments, thus making it difficult to determine an accurate offset value.